Flexible printed circuit board and corresponding secured connection method

Abstract

A flexible printed circuit board is proposed. The flexible printed circuit board includes at least one soldering element possessing a soldering face, accessible from a first face of the flexible printed circuit board, and a heating face, accessible from a second face of the flexible printed circuit board. Such a flexible printed circuit board includes a main part including the soldering element, and at least one foldable part, extending from the main part and being configured to pass: from a non-folded state, in which the at least one foldable part does not cover the heating face of the at least one soldering element; to a folded state in which the at least one foldable part covers the heating face of the at least one soldering element.

Claims

1. A flexible printed circuit board comprising: first and second faces; at least one soldering element possessing at least one soldering face, accessible from the first face of the flexible printed circuit board, and at least one heating face, accessible from the second face of the flexible printed circuit board; a main part comprising said at least one soldering element; and at least one foldable part, extending from said main part and being configured to pass: from a non-folded state, in which said at least one foldable part does not cover the at least one heating face of said at least one soldering element; and to a folded state in which said at least one foldable part covers the at least one heating face of said at least one soldering element and in which said at least one foldable part is in contact with said at least one heating face.

2. The flexible printed circuit board according to claim 1, wherein the at least one soldering element is included in a window cut out in said main part.

3. The flexible printed circuit board according to claim 1, wherein said at least one soldering element extends from an external side of said main part.

4. The flexible printed circuit board according to claim 1, wherein said at least one foldable part comprises a security mesh comprising at least one first conductive track and covering at least said at least one soldering element when said at least one foldable part is in the folded state.

5. The flexible printed circuit board according to claim 1, which further comprises means for detecting a lifting of said at least one foldable part when said at least one foldable part is in the folded state.

6. The flexible printed circuit board according to claim 5, wherein said means for detecting a lifting comprise at least one pair of first and second electrical contacts, said first electrical contact being positioned on the main part and being configured to come into contact with said second electrical contact positioned on said at least one foldable part when said at least one foldable part is in the folded state.

7. The flexible printed circuit board according to claim 5, wherein said means for detecting a lifting comprises at least one portion of a second conductive track located in said at least one foldable part and made with a conductive ink.

8. The flexible printed circuit board according to claim 7, wherein said at least one foldable part comprises a security mesh comprising at least one first conductive track and covering at least said at least one soldering element when said at least one foldable part is in the folded state, and wherein at least one portion of the second conductive track is included in said security mesh.

Description

4 LIST OF FIGURES

(1) Other features and advantages of the invention shall appear from the following description, given by way of an indicative and non-exhaustive example and from the appended drawings, of which:

(2) FIGS. 1a and 2a respectively illustrate a first face and a second face of an FPC board according to one embodiment of the invention;

(3) FIG. 1b is a magnified view of the soldering area present on the first face of FIG. 1a;

(4) FIG. 2b is a magnified view on the heating area present on the second face of FIG. 2a;

(5) FIG. 3 is a flow chart of a method for securing an FPC board according to one embodiment of the invention;

(6) FIGS. 4a, 4b and 4c illustrate the successive states of the FPC board during the implementation of the method of FIG. 3;

(7) FIG. 5 illustrates the result of the soldering and of the securing of the FPC board of FIGS. 1a and 2a to a rigid printed circuit board;

(8) FIGS. 6a, 6b and 6c illustrate different embodiments of means for detecting an attempt to access the signals exchanged between the FPC board and a circuit to which it is soldered;

(9) FIG. 7 illustrates the second face of an FPC board according to one embodiment of the invention

5 DETAILED DESCRIPTION OF THE INVENTION

(10) In all the figures of the present document, the identical elements and steps are designated by the same references.

(11) The general principle of the technique described consists of an FPC board comprising soldering elements, also called soldering pads, each possessing a soldering face, accessible from a first face of the flexible printed circuit board, and a heating face, accessible from a second face of the flexible printed circuit board. the FPC board furthermore comprises a main part, comprising elements to be soldered and a foldable part, extending from the main part and configured to pass from a non-folded state, in which it does not overlap the faces to be heated of the soldering element, to a folded state in which it overlaps the faces to be heated of the soldering elements. As a result, the soldering pads as well as the electrical signals that transit therein are no longer accessible after soldering despite the use of a window type or exposed lead type of hot-bar soldering technique.

(12) Referring now to FIGS. 1a and 2a, a description is given of a first face 110 and a second face 210 of an FPC board 10 according to one embodiment of the invention while FIGS. 1b and 2b, show a magnified view of the corresponding soldering area 105 and heating area 205.

(13) In this embodiment, the FPC board 10 has a first face 110 comprising a soldering area 105 in which there are soldering elements 106, i.e. portions of tracks of the FPC board 10 that have to be soldered, for example, to another printed circuit (for example a rigid printed circuit referenced 500 in FIG. 5 described here below).

(14) The soldering area 105 is a bare area, i.e. an area from which the layer of flexible plastic material (e.g. of the polyimide type) present on the first face 110 of the FPC board 10 has been removed so as to expose the soldering elements 106.

(15) Besides, the soldering area 105 has holes 107 passing through the FPC board 10 and opening on to the second face 210 of the FPC board 10 in its heating area 205. The soldering area 105 and the heating area 205 thus define a window comprising the holes 107 and the soldering elements 106. The heating area 205 is also a bare area, i.e. an area from which the layer of flexible plastic material (e.g. of the polyimide type) present on the second face 210 of the FPC board 10 has been removed so as to expose all the soldering elements 106. The heating area is meant to receive the thermode or hot bar with the aim of soldering the soldering elements 106 to the other printed circuit (500) on the soldering area 105 side, thus implementing a window type hot-bar soldering technique.

(16) Besides, the FPC board 10 has a foldable part 100 configured to be (i.e. capable of being and meant to be) folded on the heating area 205 so as to prevent access to the electrical signals transiting through the soldering elements 106 once soldered. More particularly, the foldable part 100 has a contact-making face 200 corresponding to a first part of the second face 210 of the FPC board 10 and meant to come into contact with a second part of the second face 210 of the FPC board 10, at the heating area 205.

(17) Besides, the foldable part 100 is demarcated from the rest of the FPC board 10 (also called a main part 103 of the FPC board 10) by a folding groove 101 herein comprising slots 102 that facilitate the folding. The folding groove 101 is used to fold the foldable part 100 on the heating area 205 in such a way that the foldable part 100 is positioned perfectly against the main part 103 of the FPC board 10. the FPC board 10 thus obtained after folding remains particularly flat, thus minimizing the space requirement of the FPC board 10.

(18) According to this embodiment, the FPC board 10 comprises a soldering area 105 and a heating area 205 that are superimposed, enabling it to be assembled (soldered) according to the technique described with another printed circuit (500). The FPC board 10 comprises, at another extremity, a card-to-card connector 215 so as to be reversibly connected to yet another printed circuit. In other embodiments, the FPC board is connected to different printed circuits in using the technique described and then has different soldering areas and different corresponding heating areas.

(19) Referring now to FIGS. 3, 4a, 4b and 4c, a description is provided of the steps of a method for securing an FPC board 10 according to one embodiment of the invention.

(20) At a soldering step E30, the soldering elements 106 of the FPC board 10 are soldered to electrical conductors (for example tracks) of a rigid printed circuit (500).

(21) More particularly, solder paste points are first of all disposed on the relevant tracks of the rigid printed circuit.

(22) Then, the soldering elements 106 of the FPC board 10 are disposed against the solder paste points, the first face 110 of the FPC board 10 being therefore oriented towards the rigid printed circuit (500), and the soldering area 105 being placed flat against the tracks of the rigid printed circuit on which the solder paste points are disposed.

(23) A hot bar is then put on the heating area 205 so that it is in contact with the soldering elements 105. Thus, the soldering compound is heated until it reaches a melting point by conduction of the heat provided by the hot bar through the soldering elements 106.

(24) Once the soldering elements 106 have been soldered to the tracks of the rigid printed circuit board, the FPC board 10 is secured during a folding step E31.

(25) More particularly, the foldable part 100 is folded on the heating area 205 by folding at the folding groove 101 (herein comprising three slots). FIGS. 4a, 4b and 4c illustrate the state of the FPC board before, during and after folding respectively. Thus, the contact-making face 200 is placed flat against the second face 210 of FPC board 10. The result of this is that the soldering elements 106 are no longer accessible from the second face 210 of the main face 103 of the FPC board 10, thereby prohibiting access to the electrical signals that travel therein. Besides, the FPC board 10 thus obtained after folding remains particularly flat, thereby minimizing the space requirement of the FPC board 10 once it is made secure.

(26) In one implementation, the folding step E31 comprises a sub-step E311 for applying an adhesive material to at least one part of an area of partial overlap of the main part 103 by the foldable part 100 before these two parts are put into contact. Thus, the contact-making face 200 of the foldable part 100 is glued to the second face 210 of the FPC board 10.

(27) In one variant which may or may not be combinable with the implementation described here above, executing the sub-step E311 for applying an adhesive material, the folding step E31 comprises a sub-step E312 for the indirect assembling of the main part 103 with the foldable part 100. Thus, the foldable part 100 is held by pressure on the second face 210 of the main part of the FPC board 10 by the use of at least one intermediate part 650 as described here below with reference to FIG. 6c.

(28) In another variant, the folding step E31 is followed by a step E32 for detecting a lifting of the foldable part 100 when this foldable part is folded on the heating area 205, the contact-making face 200 being placed flat against the second face 210 of the FPC board 10. Thus, any attempt to access the signals that transit through the solder points obtained is detected, for example by applying the means described here below with reference to FIGS. 6a, 6b and 6c.

(29) Referring now to FIG. 5, a description is provided of an FPC board 10 according to one embodiment of the invention after soldering to a rigid electronic printed circuit board (PCB) 500 and after the folding of the foldable part 100.

(30) It is thus seen that the heating area 205, and therefore also the soldering elements 106, are no longer accessible from the second face 210 of the FPC board 10 when the foldable part 100 has been folded over the heating area 205 during an execution of the step E31 of the method described here above with reference to FIG. 3.

(31) Referring now to FIGS. 6a, 6b and 6c, a description is provided of different embodiments of means for detecting an attempt to access signals exchanged between the FPC board and a circuit to which it is soldered.

(32) In a first embodiment illustrated in FIG. 6a, the foldable part 100 comprises a conductive mesh 600 that is to detect an attempt to access the signals transiting through the soldering elements 106, for example by perforation of the foldable part 100 once it is folded over the heating area 205. Indeed, the open circuit (or the short circuit) in the mesh resulting from the attempt to make a perforation can easily be detected (e.g. through means for detecting impedance variations well known to those skilled in the art).

(33) In one variant, this mesh is connected to a security mesh present on the main part 103. Thus, the detection means initially planned to detect a piercing of the security mesh present on the main part 103 (e.g. by means of a processor or a dedicated circuit to which the meshes in question are connected via a bus) are also capable of detecting a piercing of the conductive mesh 600 present in the foldable part 100: they can do so without requiring any particular adaptation. The detection of an attempt to pierce the foldable part in order to access the signals transiting through the solder points is thus easily detected, without extra cost for the complete electronic system in which the FPC board is embedded.

(34) In a second embodiment also illustrated in FIG. 6a, the contact-making face 200 comprises sections of conductive ink tracks 610 (for example using a carbon, silver or copper ink). Besides, the contact-making face is glued to the second face 210 of the FPC board 10 after soldering of the soldering elements 106, for example following an execution of the sub-step E311 for applying an adhesive material described here above with reference to FIG. 3. Thus, the tracks made of conductive ink 610 are torn off in the event of an attempt to detach the foldable part 100. As a result, any attempt at intrusion by detachment of the foldable part 100 is detected via the presence of the open electrical circuit that results from the breaking of the conductive ink tracks 610 during an execution of the detection step E32 also described here above with reference to FIG. 3.

(35) The second embodiment may or may not be combined with the first embodiment. Should the first and second embodiments be combined, the sections of conductive ink tracks 610 form part of the conductive mesh 600.

(36) In a third embodiment illustrated in FIGS. 6b and 6c (and capable of being combined with the first embodiment), the contact-making face 200 comprises first conductive devices 620 intended for cooperating with second conductive devices 630 present on the second face 210 of the main part 103 of the FCP board, herein placed in proximity to the heating area 205. Thus, when the foldable part 100 is folded, the first conductive device 620 and second conductive device 630 are put into electrical contact. To this end, the foldable part is held in place by pressure on the second face 210 of the FPC board 10. This holding is achieved for example by means of a clamping device 650 thus executing a sub-step E312 for the indirect assembling of the main part 103 with the foldable part 100 as described here above with reference to FIG. 3. Thus, in the event of an attempt to lift the foldable part 100 once it is placed on the heating area 205, the electrical contact between the first conductive device 620 and second conductive device 630 is open, thereby making it possible to detect an attempted intrusion during an execution of the detection step E32 described here above with reference to FIG. 3.

(37) Referring to FIG. 7, a description is now provided of an FPC board 10 according to another embodiment of the invention before soldering to another electronic printed circuit board and before the folding of the foldable part 100.

(38) Indeed, the embodiments illustrated in FIGS. 1a, 1b, 2a, 2b, 4a, 4b, 6a and 6b correspond to window type configurations, namely configurations where the heating area is surrounded by flexible plastic material. However, the described technique, according to any one of its different embodiments, can be applied in the same way to an exposed lead type of configuration in which the soldering elements 105 are visible at one extremity of the FPC board 10 extending from an external side 700 of the main part 103 as illustrated in FIG. 7.